Method and Component

ABSTRACT

A method for manufacturing a steel component having a flash butt welded joint. The method comprises a step of flash butt welding the weld joint by flashing and upsetting the weld and a step of supplying heat to at least the weld joint of said component after the step of upsetting the weld to increase the temperature of the weld joint, or to maintain the temperature of the weld joint at an elevated temperature.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a National Stage application claiming the benefit ofInternational Application Number PCT/SE2012/000193 filed on 26 Nov.2012, which claims the benefit of Sweden Patent Application NumberSE20110000938 filed on 20 Dec. 2011, both of which are incorporatedherein by reference in their entireties.

TECHNICAL FIELD

The present invention concerns a method for manufacturing a component,such as a bearing ring, from steel. The present invention also concernsa component manufactured using such a method.

BACKGROUND OF THE INVENTION

Flash-butt welding, or “flash welding” is a resistance welding techniquefor joining segments of metal rail, rod, chain or pipe in which thesegments are aligned end to end and electronically charged, producing anelectric arc that melts and welds the ends of the segments, yielding anexceptionally strong and smooth joint.

A flash butt welding circuit usually consists of a low-voltage,high-current energy source (usually a welding transformer) and twoclamping electrodes. The two segments that are to be welded are clampedin the electrodes and brought together until they meet, making lightcontact. Energizing the transformer causes a high-density current toflow through the areas that are in contact with each other. Flashingstarts, and the segments are forged together with sufficient force andspeed to maintain a flashing action. After a heat gradient has beenestablished on the two surfaces to be welded, an upset force is appliedto complete the weld. This upset force extrudes slag, oxides and moltenmetal from the weld zone, leaving a welding accretion in the colder zoneof the heated metal. The joint is then allowed to cool slightly beforethe clamps are opened to release the welded article. The weldingaccretion may be left in place or removed by shearing while the weldedarticle is still hot, or by grinding, depending on the requirements.

Although flash butt welding is a simple and efficient welding technique,the physical properties of a component in the vicinity of its weldjoint(s) may be adversely affected by the flash butt welding, because ofdefects, such as weld/quench cracks, which occur during and after theflash butt welding, and since the microstructure of the steel in a heataffected zone (HAZ) around a weld joint will be modified by the flashbutt welding.

SUMMARY OF THE INVENTION

An object of the invention is to provide an improved method formanufacturing a steel component having a flash butt weld joint.

This object is achieved by a method comprising the step of flash buttwelding the joint by flashing and upsetting the weld, and then supplyingheat to at least the weld joint of the component after the step ofupsetting the weld to increase the temperature of the weld joint, or tomaintain the temperature of the weld joint at an elevated temperature.

By supplying heat to at least the weld joint of a component after thestep of upsetting the weld, defects such as weld/quench cracks may beavoided or reduced. Furthermore, the microstructure in a heat affectedzone (HAZ) around the weld joint may be at least partly restored to thepre-flash butt welding microstructure so that there is no softened zonearound the weld joint in the manufactured component. The course bainitestructure normally observed in a heat affected zone (HAZ) after flashbutt welding may namely be at least party transformed by supplying heatto at least the weld joint after flash butt welding, whereby thehardness/toughness of the steel in the HAZ will be at least partlyrestored or modified to substantially match the hardness/toughness ofthe steel in the remainder of the steel, which was not adverselyaffected by heat from the flash butt welding process.

Heat may be supplied only in the vicinity of the weld joint, or to oneor more parts of the component, whereupon heat may be transferred to theweld joint, by conduction through the component itself for example. Heatis namely supplied at least to the steel that has been adverselyaffected by the flash butt welding, i.e. steel in the heat affected zone(HAZ) around the, or each weld joint of a component.

According to an embodiment of the invention the method comprises thestep of cooling the component to a temperature above the martensitestart temperature (Ms) in order to form pearlite/bainite before the stepof supplying heat to at least the weld joint of the component, and afterthe step of upsetting the weld to increase the temperature of the weldjoint of said component.

According to an embodiment of the invention the heat is supplied byheating at least the weld joint with heating means, such as inductionheating means.

According to a further embodiment of the invention the heat is suppliedby heating at least the weld joint with flash butt welding apparatus.The heat is preferably supplied by heating at least the weld joint withflash butt welding apparatus using alternating current (AC) so that thecomponent may be kept cooler than if direct current (DC) were used.

According to an embodiment of the invention the heat is additionally oralternatively supplied by insulating at least the weld joint after thestep of upsetting the weld. Thermally insulating material may beprovided at least around the weld joint to prevent, or to slow down therate of cooling of the component. A sleeve of thermally insulatingmaterial may for example be placed around the weld joint after the stepof upsetting the weld.

According to another embodiment of the invention, the method comprisesthe step of cooling the component, to room temperature for example, onlyafter the step of supplying heat to at least the weld joint.

According to a further embodiment of the invention, the method comprisesthe step of hardening at least part of the component after the heatsupplying step. The component may be cooled, to room temperature forexample, between the heat supplying step and the hardening step.

According to a further embodiment of the invention the component is aring, such as a bearing ring. The method according to the presentinvention is particularly, but not exclusively, suitable for themanufacture of large sized rings (i.e. rings having an outer diameterequal to or greater than 0.5 m, greater than 1 m, greater than 2 m orgreater than 3 m).

According to another embodiment of the invention, the steel has a carboncontent of 0.1-1.1 weight-%, preferably 0.6-1.1 weight-%, or mostpreferably 0.8-1.05 weight-%.

According to an embodiment of the invention, the steel has the followingcomposition in weight-%:

C 0.5-1.1

Si 0-0.15

Mn 0-1.0

Cr 0.01-2.0

Mo 0.01-1.0

Ni 0.01-2.0

V and/or Nb 0.01-1.0 of V or 0.01-1.0 of Nb, or 0.01-1.0 of bothelements

0-0.002

P 0-0.010

Cu 0-0.15

Al 0.010-1.0

balance Fe and normally occurring impurities.

By minimizing the silicon content, and reducing the manganese andchromium content of the steel (which are alloying elements that areeasily oxidised) to the levels indicated above, the steel will be morestable and will not be as easily oxidised during flash butt welding. Thesulphur content of the steel is reduced to an absolute minimum, wherebythe content of non-desirable non-metallic inclusions in steel that hasbeen subjected to flash butt welding will be minimized. A high level ofthrough-thickness ductility may be obtained by means of a special ladletreatment during steel making which ensures very low sulphur content anda controlled shape of non-metallic inclusions.

The phosphorus content of the steel is also reduced to an absoluteminimum in order to hinder residual or tramp elements in the steelmigrating to austenite grain boundaries when the steel is subjected toflash butt welding, which would otherwise significantly weaken the weldzone. The addition of molybdenum, nickel and optionally vanadiumprovides steel with a hardenability sufficient to enablethrough-hardening of large components (i.e. component having an outerdiameter of 500 mm or more).

The adverse effects of the unfavourable material flow that flash buttwelding creates may therefore be limited by using such steel. Using suchsteel namely provides a joined/welded component having a superiorjoint/weld since the joined/welded component does not contain areas ofstructural weakness as might otherwise occur. Such a joined/weldedcomponent therefore has a high degree of structural integrity comparedto joined/welded component that does not comprise such steel. Such steelis therefore suitable for flash butt welding and in particular for themanufacture of components intended for an application with high demandson fatigue and toughness properties, which components are to besubjected to flash butt welding during or after their manufacture.

The present invention also concerns a component that it is manufacturedusing a method according to any of the embodiments of the invention. Thecomponent may be a ring, such as a bearing ring for use in a bearingsuch as a roller bearing, a needle bearing, a tapered roller bearing, aspherical roller bearing, a toroidal roller bearing, a thrust bearing ora bearing for any application in which is subjected to alternatingHertzian stresses, such as rolling contact or combined rolling andsliding. The bearing may for example be used in automotive, wind,marine, metal producing or other machine applications which require highwear resistance and/or increased fatigue and tensile strength.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be further explained by means ofnon-limiting examples with reference to the appended schematic figureswhere;

FIGS. 1-4 show steps of a method according to an embodiment of theinvention,

FIG. 5 shows a bearing ring after a flash butt welding step according toan embodiment of the invention,

FIG. 6 shows the steps of a method according to an embodiment of theinvention, and

FIG. 7 shows a bearing according to an embodiment of the invention.

It should be noted that the drawings have not been drawn to scale andthat the dimensions of certain features have been exaggerated for thesake of clarity.

DETAILED DESCRIPTION OF EMBODIMENTS

FIGS. 1-4 schematically show various method steps of a method accordingto an embodiment of the invention. FIG. 1 shows steel 10 that is forgedto produce a steel bar 12 having two opposed ends 12 a and 12 b. A slab,bloom, or billet may be forged from an ingot weighing over 4 ton, over10 ton, over 15 ton, over 20 ton or more. At least one steel bar may beforged or cut from the slab bloom or billet. A billet is a length ofmetal that has a round or square cross-section, with an area less than230 cm2. A bloom is similar to a billet except its cross-sectional areais greater than 230 cm2. A slab is a length of metal that is rectangularin cross-section. The steel may have the following composition inweight-%: C 0.5-1.1, Si 0-0.15, Mn 0-1.0, Cr 0.01-2.0, Mo 0.01-1.0, Ni0.01-2.0, V and/or Nb; 0.01-1.0 of V or 0.01-1.0 of Nb, or 0.01-1.0 ofboth elements, S 0-0.002, P 0-0.010, Cu 0-0.15, Al 0.010-1.0 and balanceFe and normally occurring impurities.

It should be noted that the ends 12 a, 12 b of the steel bar 12 shown inthe illustrated embodiment comprise ends that form an angle of 90° to aside surface 12 c, 12 d of the steel bar 12. A steel bar 12 may howevercomprise an end 12 a, 12 b that forms an angle greater or less than 90°to a side surface 12 c, 12 d of a steel bar, a steel bar 12 may namelycomprise diagonally sloping ends. Furthermore, the ends 12 a and 12 b ofthe steel bar 12 need not necessarily have a flat surface.

At least one part of at least one surface 12 a, 12 b, 12 c, 12 d of thesteel bar may be carburized prior to flash butt welding. For example,the opposed ends may be uniformly or non-uniformly carburized to form acontinuous or non-continuous carburized layer using any conventionalmethod in which the steel bar is heated in the presence of anothermaterial that liberates carbon as it decomposes and then cooled rapidlyby quenching.

FIG. 2 shows a single steel bar 12 that has been formed into an openbearing ring 14. It should be noted that each of a plurality of steelbars 12 may alternatively be formed into a ring segment, whereby two ormore ring segments may then be flash butt welded together to form abearing ring 14 comprising two or more weld joints.

FIG. 3 shows the ends 12 a, 12 b of an open bearing ring 14 being flashbutt welded together. The ends 12 a, 12 b of the open bearing ring 14are clamped and brought together at a controlled rate and current from atransformer 16 is applied. An arc is created between the two ends 12 a,12 b. At the beginning of the flash butt welding process, the arc gap 18is large enough to even out and clean the two surfaces 12 a, 12 b.Reducing and then closing and opening the gap 18 creates heat in the twosurfaces 12 a, 12 b. When the temperature at the two surfaces 12 a, 12 bhas reached the forging temperature, pressure is applied in thedirections of block arrows 20 in FIG. 3 (or a moveable end is forgedagainst a stationary end). A flash is created between the two surfaces12 a, 12 b, which causes any carbon in the welding area to flow radiallyoutwards from the surfaces 12 a, 12 b towards the inside and outsidesurfaces 12 c, 12 d of the bearing ring, resulting in a clean weldjoint. After flashing, an upset force is suddenly applied to completethe weld. This upset force extrudes slag, oxides and molten metal fromthe weld zone leaving a welding accretion in the colder zone of theheated metal.

According to an embodiment of the invention the welded bearing ring iscooled to a temperature above the martensite start temperature (Ms) inorder to form pearlite/bainite after the step of upsetting the weld toincrease the temperature of the weld joint of said component.

FIG. 4 shows that, after the flashing, upsetting and cooling steps, heat22 is supplied to the weld joint 24 of the component to increase thetemperature of the weld joint 24 or to maintain the temperature of theweld joint 24 at an elevated temperature. The heat 22 may be supplied byany suitable heating means, such as by induction heating means.Additionally, or alternatively, heat 22 may be supplied using the flashbutt welding apparatus itself, using alternating current (AC) forexample. Alternatively, or additionally the weld joint 24 may beinsulated by providing thermally-insulating material at least around theweld joint 24. For example, a sleeve of thermally insulating materialmay be placed around the weld joint 24. Heat may be supplied to the weldjoint 24 so that the temperature of the weld joint is kept at atemperature of about 900° C. for at least 5 minutes.

At least part of the welded component may be subjected to a post-weldingheat treatment, such as carburizing, after the heat supplying step inorder to increase its surface hardness, wear resistance and/or fatigueand tensile strength. Carburizing is a heat treatment process in whichan iron or steel component is heated in the presence of another materialthat liberates carbon as it decomposes. The outer surface of thecomponent will have a higher carbon content than the original material.When the iron or steel component is cooled rapidly by quenching, thehigher carbon content on the outer surface becomes hard, while the coreremains soft (i.e. ductile) and tough.

Alternatively, the welded component may be cooled after the heatsupplying step, in a water-, oil- or polymer-based quench for example.

Any welding accretion 26, containing slag, oxides and/or molten metalfor example, (shown in FIG. 5) which accumulates on the inner and outersurfaces 12 d and 12 c of the welded bearing ring may be removed byshearing or grinding for example.

FIG. 6 shows the steps of a method for manufacturing a component fromsteel according to an embodiment of the invention. The method comprisesthe steps of flash butt welding the component by flashing and upsettingthe weld, cooling the component to a temperature above the martensitestart temperature (Ms) in order to form pearlite/bainite and thensupplying heat to at least a weld joint of the component to increase thetemperature of the weld joint, or to maintain the temperature of theweld joint at an elevated temperature. The component is not allowed tocool substantially between the steps of upsetting the weld joint andsupplying heat to at least the weld joint of the component, i.e. thecomponent is not cooled to room temperature, for example, before heat issupplied to at least the weld joint. After heat has been supplied to theweld joint for a predetermined amount of time, at least part of thecomponent may be subjected to hardening heat treatment for example.

FIG. 7 shows an example of a bearing 28, namely a rolling elementbearing that may range in size from 10 mm diameter to a few metresdiameter and have a load-carrying capacity from a few tens of grams tomany thousands of tonnes. The bearing 28 according to the presentinvention may namely be of any size and have any load-carrying capacity.The bearing 28 has an inner ring 30 and an outer ring 32, one or both ofwhich may be constituted by a ring according to the present invention,and a set of rolling elements 34. The inner ring 30, the outer ring 32and/or the rolling elements 34 of the rolling element bearing 28, andpreferably all of the rolling contact parts of the rolling elementbearing 28 are manufactured from steel that comprises 0.20 to 0.40weight-% carbon.

A component manufactured using a method according to an embodiment ofthe invention, in which heat has been supplied at least to the weldjoint(s) of the component after flash butt welding, will have a smallerheat affected zone (HAZ) than a corresponding component manufacturedusing a conventional method in which heat is not supplied to thecomponent after flash butt welding, but in which the component is cooledwithout using any thermal insulation. Such a component will thereforehave improved and/or more uniform physical properties as compared with acomponent manufactured using said conventional method.

Further modifications of the invention within the scope of the claimswill be apparent to a skilled person.

1. A method for manufacturing a steel component having a flash butt weldjoint, the method comprising steps of: flash butt welding the joint byflashing and upsetting the weld, and supplying heat to at least the weldjoint of said component after the step of upsetting the weld to increasethe temperature of the weld joint, or to maintain the temperature of theweld joint at an elevated temperature.
 2. The method according to claim1, further comprising a step of cooling said component to a temperatureabove the martensite start temperature (Ms) before said step ofsupplying heat to at least the weld joint of said component and afterthe step of upsetting the weld to increase the temperature of the weldjoint of said component.
 3. The method according to claim 1, whereinsaid heat is supplied by heating at least said weld joint with a heatingmeans.
 4. The method according to claim 3, wherein said heating meanscomprises induction heating means.
 5. The method according to claim 1,wherein said heat is supplied by heating at least said weld joint withflash butt welding apparatus.
 6. The method according to claim 5,wherein said heat is supplied by heating at least said weld joint withflash butt welding apparatus using alternating current (AC).
 7. Themethod according to claim 1, wherein said heat is supplied by insulatingat least said weld joint after the step of upsetting the weld.
 8. Themethod according to claim 1, further comprising a step of cooling saidcomponent only after said step of supplying heat to at least said weldjoint.
 9. The method according to claim 8, further comprising a step ofhardening at least part of said component after said heat supplyingstep.
 10. The method according to claim 1, wherein said steel has acarbon content of 0.1-1.1 weight-%.
 11. The method according to claim 1,wherein said component is a ring.
 12. The method according to claim 11,wherein said ring is a bearing ring.
 13. The method according to claim11, wherein said ring has an outer diameter equal to, or greater than0.5 m.
 14. The method according to claim 1, wherein said steel has thefollowing composition in weight-%: C 0.5-1.1% Si 0-0.15% Mn 0-1.0% Cr0.01-2.0% Mo 0.01-1.0% Ni 0.01-2.0% at least one of V and Nb 0.01-1.0%of V or 0.01-1.0% of Nb, or 0.01-1.0% of both elements S 0-0.002% P0-0.010% Cu 0-0.15% Al 0.010-1.0%, and a balance of Fe and normallyoccurring impurities.
 15. A component having a flash butt weld joint,said component is manufactured by: flash butt welding the joint byflashing and upsetting the weld, and supplying heat to at least the weldjoint of said component after the step of upsetting the weld to increasethe temperature of the weld joint, or to maintain the temperature of theweld joint at an elevated temperature.
 16. The component according toclaim 15, wherein said component is a ring.
 17. The component accordingto claim 16, wherein said component is a bearing ring.